Not sure if this helps, but there is a paper by Lovesy, "Development of the Rolls-Royce Merlin from 1939 to 1945," that gives the following for the performance of the two stage supercharger at 30,000 feet:

62% efficiency at a compression ratio of 7.2 (RMS14?)
65% efficiency at a compression ratio of 8.2 (experimental design?)

there is a graph showing overall efficiency vs pressure ratio (figure 11), but unfortunately, it is quite small and not terribly clear on the copy I have.